A cover member for a substrate supporting a biological sample comprises first and second opposing ends, first and second opposing surfaces, a void in the second surface which, when juxtaposed with a substrate, forms a chamber, and a fluid inlet toward the first end and in fluid communication with the void. The void is bounded by void walls having one or more contoured regions for enhancing fluid movement within the chamber. A treatment module for a biological sample comprises the cover member, a support surface for a substrate bearing the biological sample and clamp means operable to releasably retain the cover member in juxtaposition with the substrate for an incubation period. A method for incubating the biological sample with one or more reagents uses the cover member.